Vapor compression evaporation



June 28, 1960 R. v. KLEINSCHMIDT 2,942,657

VAPOR COMPRESSION EVAPORATION Filed May 28, 1957 2 Sheets-Sheet 1 Fig. I A B c CONCENTRATED PRODUCT CONDENSATE OUTLET Fig.2 :2 CONDENSER E l PRESSUREJG- FBSJ. GAUGE F I I 1 I 1 4 H I O 11 |1||2||!13i!l|||||!||16||ll7ll |18l|| llll 1 4 5 WEIGHT RATIO, SOLVENT/SOLUTE INVENTOR ROBERT V. KLEINSCHMIDT '7 fTgRNa s June 28, 1960 R. v. KLEINSCHMIDT 2,942,657

VAPOR COMPRESSION EVAPORATION Filed May 28 1957 2 Sheets-Sheet 2 l2 CONDENSER I PRESSURE, RSI. I GAUGE I F 1 i I l 1 1 I i I 1 .M 1 a 1----- i l I 2 O I I 1 1 1l|1II1|1 |1| 11|| 1:11 llll l1|| |1|| Ililillll 0 l 2 3 5 WEIGHT RATIO Hg. 3 SOLVENT SOLUTE Fig. 4 CONDENSER PRESSURE, RS1. GAUGE 30 1 11 1 1 1111 1 1 20 f 6 B, RISE, 0F

O 1 1 1 l 1 1 1 1 1 1 1 I I 1 l 1 I 1 1 1 I 1 1 I 0 IO 20 3O 4O 5O 6O CONC. OF NGOH INVENTOR. ROBERT V. KLEINSCHMIDT United States Patent VAPOR CONIPRESSION EVAPORATION Robert V. Kleinschmidt, Stoneham, Mass, assignor to The Bowen Corporation, Cambridge, Mass., a corporation of Massachusetts Filed May 28, 1957, Ser. No. 662,285

3 Claims. (Cl. 159-47) the source of energy to efiect further evaporation. In

a typical operation the solution is maintained in an evaporation zone from which vapor of solvent is removed. This vapor is compressed to an elevated pressure, whereby its temperature is increased, and is then brought into out-of-contact heat exchange relation with the body of solution in the evaporation zone. Because of its elevated pressure, the vapor may be condensed at a temperature above the boiling point of the solution in the evaporation zone, and the heat of evaporation may be utilized to effect further evaporation. The process may thus be maintained continuously by utilizing the energy of compression as the equivalent heat source. By transferring the sensible heat of the effluent streams of both condensed evaporate and concentrated effluent to the incoming solution, the total energy input relative to the amount of solvent removed from the solution may be maintained at an extremely low level.

It is of course necessary to such a process that the solvent vapor be compressed sufiiciently to elevate its condensation temperature to above the boiling point of the solution. Where the solution is of high solute concentration the vapor must be more highly compressed than when evaporation is effected from a solution of lower concentration.

When vapor compression evaporation is carried out as a continuous process the product is continuously withdrawn from the body of evaporating solution. That solution and the product must both therefore be maintained at substantially the same concentration. This fact has heretofore limited the advantages of vapor compression processes to cases where product of low concentration is desired.

Where a final solution of high concentration is desired its boiling point is generally so much higher than the boiling point of the pure solvent that the mechanical work of compressing the solvent vapor sufiiciently to elevate its condensation temperature to above the boiling point of the solution is excessive. Accordingly, prior art processes of vapor compression evaporations have not proved successful where a highly concentrated prodnet is to be produced.

I have however discovered that such a large energy input is not necessary to produce a high degree of concentration. The object of this invention is to provide a continuous process of vapor compression evaporation 2,942,657 Patented June 28, 1960 2 which requires only a fraction of the energy input of those processes as practiced .in the past.

According to my present invention the evaporation process is carried out in stages in each of which only a fraction of total amount of solvent is removed. As a result, in each stage prior to the last one the concentration and boiling point of the solution are maintained lower than the concentration and boiling point of the final product. Consequently the amount of vapor re moved in each stage of the'process prior to the last one is compressed to a lesser extent than it would be if all the vapor were removed in one step. The bulk of solvent is accordingly removed from a solution boiling at a lower temperature than the final temperature, and its vapors need not be so highly compressed. Only in the last stage is the vapor compressed to the extent necessary to elevate the condensation temperature to above the boiling point of the final product, but, only a fraction of the total vapor is handled in that stage. I Y

My present invention represents a significant improvement in vapor compression evaporation processes and renders them increasingly useful where product of high solids content is to be produced. One typical application of vapor compression evaporation that is greatly improved by this invention is the recovery of solids from waste sulfite paper liquors, .where a product of high solids content is desired.

This invention accordingly features a plurality of evaporation stages through which the solution to be evap' orated is flowed continuously from one to the next. In the last stage the solution is maintained at its finalconcentration while in the preceding stages the concentration is lower. 7 In each stage vapor is withdrawn from the. solution and is then compressed and passed in outof-contact heat exchange relation with the evaporating solution in that stage. Since the flow rates of solution and the rate and degree of compression in each stage are all individually controllable, each stage may be operated substantialy independently of the other stages.

In its preferred embodiment the stages are controlled so that the energy input to each one is the same, that is to say, that the product of the amount of solvent times the pressure increase is the same for each stage.

The invention also features a novel manner of recovering the sensible heat from the efiluent condensed evap orate by transferring it to the incoming solution. An efiicient heat balance may accordingly. be easily maintained.

The invention is described in greater detail below with reference to a presently preferred embodiment which has been selected for purposes of illustration, wherein reference is made to the drawings in which:

Fig. 1 shows schematically a three stage vapor-compression evaporator embodying this invention;

Fig. 2 is a graphical presentation of the work input required to effect a given degree of concentration in which a single stage process is compared with a twostage process; V

Fig. 3 is a graphical presentation of the work input required to efiect a given degree of concentration in which a single stage process is compared with a threestage process; and

'Fig. 4 is a graph correlating boiling point rise with concentration of a sodium hydroxide solution, showing also the degree of compression required to utilize the compressed vapor as the heat source at various thermal potentials (temperature difference between condensing compressed vapor and boiling liquid).

A typical system for carrying out this invention, utilizing three stages of vapor compression evaporation, is shown schematically in Fig. 1. The system is one which mayp-for' example, be used'toconcentrat: waste soda base blackliquor froma-paper pulp The system consists of. three separate evaporator-condenser stages which are designated generally asA', B and Cl "Each stage comprises an evaporator-condenser unit in the general form oflan evaporator vessel containing compression, equalityof energy input may be achieved even though lesssolvent is pumped,

Optimum operative conditions for any given operation can be shown graphically, as in Figs. 2 and 3 which illusa 'condenser 12 byjwhich hot vapor may be brought into 7 outi-of contact heat exchangerelation with solution in the evaporator.- i Each evaporator 10 has a solution inlet l 3='a t' its lower end and an overflow outlet pipe 14 drainin'g' fromthe upper surface of the solution' and maintaining a substantially constant solution level inside the V evaporator. A vap'or line 15*leads mm the top of the evaporatorflltl" to the inlet of a compressor 16 and a compressed'vapordine 18 leads firomthe outlet of the compressor to the inlet of'the condenser 12. The outlet fiom each condenser 12 connects through a valve 19 with one of a-"scries' preh'eater exchangers; The condensate from the 'lastlunit C enters an exchanger 20, the condensate from 'theintermedi-ate unit B, together'with'the effluent condensate from theexchanger 20, ente'rs'an exchanger 22,- aud'the condensate'from the first unit A,'together enter sanexchanger ;24. The eflluent. condensate from each of the exchangers 20, 22 and 24 discharg'es through a-pr essure valve 25 by which the pressure: of the conden= sate may be maintained suificiently high to prevent flashring; the condensate is discharged from" the 'ex changer 24. V 7

The; solution to be concentrated is first preheated to near or above its boiling point by passing it in series through the exchangers 24, 22 and 20, in out-of-contact heat 'exchange' relation with increasingly hotter con'densate and is 'then fed into the evaporator-condenser unit I A. iThe' overflow from'unit' A'is fed to unit 'B and the overflow from unit B is fed to the final concentration unit C, from which product is withdrawn.

j In eachrof the units solvent is removed by operation of the :vapor pumps 16. V Vapor is compressed in the pumps until its condensation temperature'is abovethe boiling point of the'solution in the evaporator by the desired therm'al potential. it When so compressed the vapor is j superheated as 'it leaves the pump.' The vapor'is'fdischarged-into the'condensers-lz and is there'condensed by the transfer of its heat to the evaporating solution. 'This transfer results in the' generation of additional vapor which in is compressed and fed'to the condenser. V In each successive uniHhe. solution is maintained at a unit is higherthan in the preceding unit. Preheating of the; fresh solution in the manner described utilizes the ever increasing condensate temperature by progressively V bringing the fresh solution into heat exchange relation with mcreasmgly hot fluid in a manner such that the fresh solution'is' preheated very nearly to is boiling point, or in many cases to above its boilingpoint.

Because of the eflicient recoveryof the sensible heat the condensate, a very favorable heat balance may be maintained and':the process sustainedby the energy of a compression supplied to the compressors 16 jln each unit,'.the solution is maintained at a concentrati'oriselected toiachieve the'desire final concentration villi-b10113 energy input, and this concentra nt o'n is' maintained by controllingth e compressor 16 to V removethe requisite amountof solvent Minimum total energy input is *achieved whentheenergy input to each of the several'stages is-equal, and in general this condito remove'le'ss-solvent*than'the preceding stage.- Since each success-ive stage 7 requires 'an increased degree "of trate a 2 and 3-stage evaporation process respectively by correlating condenser pressure necessary to' achieve a condensation temperatureabovei the boiling point of the solution with the concentration of the solution expressed as the weight ratio of solvent to dissolvedsolids. On 1 such a graph,- horizontal distance represents the weight of solventremoved from a unit weight o f'solute between any given concentration limits, and vertical distance, at the high concentration limit'represents the amount by which the vapors of'solvent must be compressed if they are to be used efiectively asjthehea't source fora boiling solution of that concentration. V

Fig. 2 may be taken as representative of a typical operation, such as theevaporation of waste soda base black with the effluent condensate from the exchanger 22,

liquor, to illustratethe amount of work required tdconce'ntrate the solution from an initial concentration of' l0% solute to a final concentration ,of 50% solute. 1! These limits correspond-to-an initial ratio of solventeto-soluteof. 9 and a final ratio of solvent-to-solutc of l, and theremoval of 8 parts by Weight of solvent per par-tof solute.

The final evaporator condenser must be operated with the solution in it at the final concentration and the temperature in that unit must correspond to the boiling point to the 8' pounds per squarejinchlnecessary to operate' concentration 'g'reater' than that in the next preceding unit, and consequently at-a correspondingly higher temperaturei, Asa result, the pressure inthe condenser'of "each unit must be greaterthan'inthe preceding unit. in a order that thecondensation temperature is correspond: ingly'higher,' and the condensate temperature 'from each of the final solution. The vapor in the final unit 'must accordingly be compressed to about 8 p.s.i.s, asindicated concentration'is carried out in two or'more stages the bulk of thel solvent may be removed with considerably less work; Againreferrin'g to'Figl 2, if 6 units of solvent are removed, in the firstjstage to bring the solute concentration to 25 parts by weight ('s'OlVent-to-solute'iratio of 3).,the vapor initially refrnoved. needibe compressed to only about 2.7 pounds p'er square inch, an diu the final stage it is necessary to compress only 2 units'ofj vapor that stage. 7 With the 2-stage ,ope'rationfthe total of work 'may be represented by the area under the line FG plus the rea under the line HK; the amountof .worlr 'represented by the area GHKI is saved. V,

Still "further savings may be effected byfurther'in'creas ing the number of stages, as suggested by Fig. 3 in which a 3-s-t'ag'e operation is compared with 'a single-stage operation. In this case-the total work is shownlbythearea under the line FL plus the areaunder MN plus the area 7 OP,-; and the work saved is -represented by: the area LMNOPI. V is V 7 Optimum operating conditions in the process' of this inventionare achieved if the totalconcentration' efiected by any two units is shared such'thatboth unitsperform the same amount of work; I have found thatlapproximately equal divisionflof the work will be achieved if between any two stages the concentration ofthei eflluent "of the'first of these two stages'is equal to the square 7 tion-iis;achieved-wheneach successive stage isoperated root of the product ofthe initial concentrationof the feed to the first stage times the finah concentr ation ofjthe etfiuent from the second Stage each expressed. as the weight ,ratio of solvent to solute. in'the above example Optimum operation is achieved in the Z-stage system when the concentration of the product in thefirst unit is 3. parts of solvent per part of solute,;or a weight concentration 'of'solute of 25%. The same relationship 5' also applies to any two adjacentstages'of ai3-sta'gersystem.

The overall'division is achieved whengthe first stage is min-- t "1-,.

The concentration of the solution from the second stage of the 3-stage system may be expressed by the formula:

etc.

In Fig. 4 the boiling point rise is plotted as a function of concentration of a sodium hydroxide solution, and condenser pressure is shown correlated with boiling point. This graphical presentation is useful in selecting operating conditions for a multi-stage vapor compression evaporation of this particular solution between any desired concentration limits. To assist in the graphical analysis a number of lines of unit slope spaced apart by a distance corresponding to a temperature difference of 2 F. are plotted so that the pressure rise necessary to attain a condensation temperature any desired thermal potential above the boiling point of the solution may be easily selected at any desired product concentration.

To establish operating conditions in evaporating a sodium hydroxide solution in accordance with this in vention, the concentration of the solution in each of the stages may be selected for near optimum operation from the formulae above. From the concentration, the boiling point rise over that of pure solvent may then be determined from the lower graph of Fig. 4, which correlates boiling point rise with concentration. The amount of compression of the vapor necessary to attain a condensation temperature equal to the boiling point may now be obtained at the intersection of the upper graph of Fig. 4, which correlates boiling point rise with pressure rise, and a line through the operating point and parallel with the abscissa. With the aid of the parallel unit slope lines, a vertical distance corresponding to any desired thermal potential may be measured; a corresponding pressure rise to attain that potential may then be read from the upper graph.

Operating conditions in the two-stage concentration of a 5 percent by weight sodium hydroxide solution to a concentration of 50 percent by weight, with each stage operating under a thermal potential of 6 Fahrenheit degrees, may be determined as follows:

(1) Concentration of product in first stage:

X 19 parts solvent to solute X =l part solvent to solute X =(l9 1) =4.36=l8.6%

(2) Boiling point rise, first stage: From Fig. 4, at point a=4.7 F.

(3) Pressure rise in first stage to attain a thermal po tential of 6 F.:

be represents AT=6 d represents corresponding pressure rise=3.5 p.s.i.

(4) Boiling point rise, second stage: From Fig. 4, at point 2:20 F.

. .6 (5) Pressure rise in second stage to attain a thermal potential of 6 F.:

representsAT=6 .g It represents corresponding pressure rise=9.4 p.s.i.

According to this invention therefore two-stage evaporationof a hydroxide solution requires that the 1464 parts of solvent-per part of sodium hydroxide removed in the' first stage be compressed 3.5 p.s.i. In the second stage only the remaining 336 parts of solvent are compressed 9.4 p.s.i; In a single stage of vapor compression evaporation, on the other hand, the-entire 18 parts of solvent would have to be compressed the full 9.4 p.s.i. necessary to attain a thermal potential of 6 Fahrenheit degrees.

This invention has been described with specific reference to its preferred embodiment, but it is contemplated that modifications will readily occur to those skilled in the art, and that such may be made without departing from the scope of this invention.

Having thus disclosed my invention and described in detail representative examples of its preferred embodiments, I claim and desire to secure by Letters Patent:

1. The continuous method of increasing the concentration of solute in a solution from an initial concentration to an elevated concentration by evaporating volatile solvent from said solution comprising continuously flowing said solution of initial concentration initially in succession through a first heat exchanger and through a second heat exchanger and then through a first evaporation zone and continuously withdrawing a first fraction of vapor from said solution in said first evaporation zone, said flowing and withdrawing being at rates correlated to maintain the solution in said first evaporation zone at an intermediate concentration, mechanically compressing said vapor to elevate its condensation temperature to above the boiling point of said solution of intermediate concentration, passing said compressed vapor in out-ofcontact heat exchange relation with said solution of intermediate concentration in said first evaporation zone and causing said compressed vapor to condense therein to form a first fraction of condensate and flowing said first traction of condensate through said first heat exchanger in out-of-contact heat exchange relation with said solution of initial concentration to preheat same, continuously flowing said solution of intermediate concentration through a second evaporation zone and continuously withdrawing a second fraction of vapor from said solution in said second evaporation zone, said flowing and withdrawing being at rates correlated to maintain the solution in said second evaporation zone in said elevated concentration, mechanically compressing said second fraction of vapor to elevate its condensation temperature to above the boiling point of said solution of elevated concentration, passing said compressed second fraction of vapor in out-of-contact heat exchange relation with said solution of elevated concentration in said second evaporation zone and causing said second fraction of vapor to condense therein to form a second fraction of condensate and flowing said second fraction of condensate through said second heat exchanger in out-of-contact heat exchange relation with said preheated solution of initial concentration, and finally removing said solution of elevated concentration from said second evaporation zone.

2. In the continuous method of increasing the concentration of solute in a solution from an initial concentration to an elevated concentration by evaporating volatile solvent from said solution and comprising continuously flowing said solution of initial concentration through a first evaporation zone and continuously withdrawing a first fraction of vapor from said solution in said first evaporation zone, said flowing and withdrawing being at rates correlated to maintain the solution in said first evaporation zone at an intermediate concentration, mechanically compressing said vapor to elevate its condensation temperature to above-the boiling pointof saidsolution of intermediate concentration, passing said, compressed 'vapor in out-of-contaot heat exchange relation with'said solution of intermediate concentration'in saidffirst evapf oration-zone and' causing said compressed Vapor 110 conens there n cont nuousl fl win s d q t qneo interme iate wnce t t qnt j u h a. secon evap rat z and cqmi g ly with r win ar eqqndf c ionp vapor from a d; s lutiq dn a d s n evaporat o zone said 7 flowi a withdrawing be n v a rate w e t t mai n t s o in s ondnqr n. o w atsai elevated 'fconcentration, v mechanically; compressing said se ond f ac ion o v po to el a i q e a t perature ,tdabdye the boilingpoint of said solution of elevated concentration, passing said' compressed second fraetion' of vapor inaout-of-contact heat exchange relation with said solution of elevated. concentration-iii said second eyappration z one and causing'saidsecond fraction of yaporto condense thereiniand finally removing said solution of elevated concentration from said second evap oration zone: controlling the amount of-said withdrawal said withdrawal o fjsaid second fractionof vapor such as 1 to maintain .the intermediate concentration 7 expressed in 'terms; of the,weight'ratio of solvent-to-solute about, equal energy utilized in eornpressing said first fraction of vapor.

7 References Cited in the'file of patent OTHER'REFERENCES Heat Transfer and Eyaporationjw. L. Badger, published by The ChemicalC atalog CoQI'nc. NYC. in 1926. Pages 155, 263-266.

Evaporation by Webre and Robinson, pub. by Chemical -Catalog Co Inc. NYCiin" 1926,-page 289. i 

